Individual, Place, and Time Criteria for Diagnosis and Testing

Individual.—All ages and sizes of adult (sessile) coral are affected. Adult coral may be aged by annular rings in a manner like tree rings (Buddemeier, 1976). No data are available regarding larval stages. Larger colonies may survive longer and be therefore more likely to be observed, but larger size is not protective overall. Recently dead coral in an area experiencing an outbreak may be suspected as mortality caused by SCTLD (Aeby and others, 2019; Precht and others, 2016).

Place.—SCTLD has been documented along Florida’s Coral Reef (Florida Reef Tract) from Martin County south to Key West and now the Dry Tortugas (Muller and others, 2020; FDEP, 2018). Outbreaks of a disease with lesions grossly similar to SCTLD have also been confirmed in the Caribbean and Bahamas including Jamaica, Quintana Roo (Mexico), St. Maarten, St. Thomas, Dominican Republic, Turks and Caicos Islands, Saint-Martin, Belize, St. Eustatius, St. John, Puerto Rico, Grand Bahamas, Grand Cayman, United States and British Virgin Islands, Cayman Islands, Guadeloupe, and St. Lucia (Dahlgren, 2020; Marks and Lang, 2018). Honduras, Martinique, and Dominica were recently added to the Atlantic and Gulf Rapid Reef Assessment (AGRRA) dashboard (Kramer and others, 2019). Note that this disease is continuing to spread through the wider Caribbean and will almost certainly be present in new areas as time passes. Active surveillance of Pacific reefs is important because the Panama Canal could present a conduit for SCTLD into the Pacific, and susceptibility of Pacific species is unknown.

Time.—The SCTLD outbreak was noted in 2014 and was associated with unusually high temperatures, outflow of semi-treated sewage, and a dredging event in Miami harbor (Miller and others, 2016), and the disease has been consistently present in the Florida Reef Tract since that time. No clear pattern with respect to season has been identified (Aeby and others, 2019; Precht and others, 2016), but temperature increases have been associated with a decline in incidence and disease progression (Meiling and others, 2021; Williams and others, 2021).

Field Criteria for Diagnosis

History and clinical signs.—Diagnostically compatible illness may present with multifocal to coalescing sharply demarcated acute to subacute areas of tissue loss that may begin at the base of the colony and spread upwardly (apically) or appear randomly distributed. The area of tissue loss represented by bare skeleton may be bordered by a narrow band of bleached tissue but lacks any pigmented band at the tissue-loss margin. Skeleton at the tissue-loss margin is usually acute, appearing white and intact (depending upon species), although algae and fungal mats denoting older, subacute, or chronic lesions may accumulate on older areas of denuded skeleton.

Rapid rate of tissue loss, 5 to 40 square centimeters per day (Sharp and Maxwell, 2018), is highly suspicious but not pathognomonic for SCTLD (Bruckner, 2016). Affected individuals typically die within weeks to months depending on species (FDEP, 2018). The presence of multiple affected species on a reef consistent with the previously observed susceptible species also increases suspicion, as do proportionally large areas of tissue loss relative to colony size. Species that are more highly susceptible including boulder brain coral, pillar coral, elliptical star coral, grooved brain coral, maze coral, symmetrical brain coral, and knobby brain coral are typically affected first; and colonies may be discovered with complete mortality even on initial discovery. Species with intermediate susceptibility, including boulder star coral, mountainous star coral, large-cup star coral, smooth star coral, blushing star coral, and starlet coral show signs on average 1 month later and may die over months to years. Starlet coral is an exception and may show signs earlier, although progression may not match other species (FDEP 2018, Aeby and others 2019).

Other.—In Florida and aquaria experiments, induction of tissue loss appears to spread within the water column and through currents (Muller and others 2020), although natural water currents cannot explain spread throughout the Caribbean. Factors such as human activity and possible pathogen(s) transmission via contaminated bilge water and recreational or commercial equipment used in activities such as diving, boating, or fishing have all been suspected to play a role (Dahlgren, 2020). Because gross lesions alone are not diagnostic, SCTLD cannot be ruled in or out of a particular site distant from other affected areas if it meets current gross criteria of the case definition (refer to the “Laboratory Criteria for Diagnosis” section).

In laboratory studies, apparently healthy coral species exposed to affected wild corals by direct contact or indirect exposure developed the disease, so wild-collected coral are also a potential source of transmission (Aeby and others, 2019). However, to date no evidence of infectious agents consistently associated with SCTLD have been documented on light microscopy (Landsberg and others, 2020). Bacterial causes have been hypothesized, but no bacteria have been found consistently associated with lesions. Virus-like particles have been observed within endosymbionts of both SCTLD affected and apparently healthy exposed coral, causing lysis and death of affected endosymbionts, but more data are needed to conclusively tie these to disease (Work and others, 2021).

Laboratory Criteria for Diagnosis

Gross examination.—Diagnostically compatible findings may include focal or multifocal to coalescing patches of bleaching and tissue loss, or tissue loss beginning at the base of the colony that may be bordered by a narrow band of bleached tissue. For S. siderea, initial lesions may be purple or gray discolored tissue around the oral cavity rather than white denuded skeleton (fig. 1A). More chronic areas of denuded skeleton often develop a mat of fungal and (or) algal overgrowth, whereas areas of acute loss abut white, bare skeleton (Paul and others, 2019) (fig. 1B).

Histopathology.—The histological presentation varies among species, and many affected species have not been examined microscopically. However, of those that have been examined (n=8), commonalities are described in this document and in (Landsberg and others, 2020). The tissue loss margin is often too autolyzed to be diagnostically useful, and apparently normal tissue on the affected colony distant from a lesion may be more diagnostic; therefore, biopsies should be taken from grossly normal areas of affected colonies as well as along the tissue loss margin. Histologic lesions can be seen in tissue far from the tissue loss margin in the coenenchyme or deeper in the polyp (fig. 2D).

Histological lesions are first observed and most severe in the basal body wall of the gastrodermis and calicodermis, and center on the endosymbionts (Symbiodiniaceae) (figs. 3B, 3C). Endosymbionts show vacuolation, gray to brown discoloration and enlargement of the symbiosome (figs. 3D, 4D, 5D), or features of necrosis including deformation, cytoplasmic hypereosinophilia or hypopigmentation, swelling or shrinkage, nuclear pyknosis, karyorrhexis, or pallor with nuclear loss (ghost cells) (figs. 6D, 9D). In and around foci of affected algal symbionts, gastrodermis, especially at the basal body wall but also extending into the surface body wall, mesenteries, and cnidoglandular band, shows varying degrees of vacuolation attributed to increased mucocytes or cell swelling. There may also be characteristic discrete foci of lytic necrosis that have loss of cell detail (fig. 6B), disruption of tissue architecture, loss of endosymbionts, support cell necrosis, and detachment from the mesoglea (cleft formation) (figs. 7B, 8C). The mesoglea is often eroded (figs. 9C, 9D), pale, and displays irregular staining (fig. 8D, 9D). The calicodermis may be segmentally cuboidal to columnar (metaplastic) or sloughing and necrotic (figs. 8B, 8C).

In large-cup star coral and symmetrical brain coral, rhomboidal, 1- to 10-micrometer intracytoplasmic Periodic acid-Schiff (PAS)-positive and eosinophilic inclusions, or crystalline inclusion bodies (CIBs; presumably within the cytoplasm of mucocytes), have been seen in the basal body wall gastrodermis, sometimes within or at the periphery of lesions (including degenerating mucocytes); however, these are not always seen, and they may also be present in apparently healthy tissue (Landsberg and Kiryu, 2016; Landsberg and others, 2020). Basal body wall, calicodermis, and gastrodermal lesions are often present in the absence of surface or epidermal lesions, and mesoglea next to foci of necrosis is often disrupted or expanded with clear space (figs. 2F, 5D). Histologic lesions may be seen in coral from an affected reef that do not show gross signs of tissue loss.

Bacteria are not typically associated with lesions in hematoxylin and eosin-stained sections, but suspect bacteria in basal and surface body wall gastrodermis and cnidoglandular bands have been found using Giemsa, thionin, or Gram stains, some of which also bind to fluorophore-tagged eubacterial probes (EUB338-I, EUB338-II, EUB338-III) using onsite hybridization procedures (Esther Peters, George Mason Unversity, oral commun., 2020). Endolithic fungi or algae may be noted as are intracellular coccidia in the gastrodermis, but these are not a consistent feature, and their significance is uncertain.

Diagnostic test(s).—At present, the only diagnostic test to confirm SCTLD is light microscopy of paraffin embedded tissues stained with hematoxylin and eosin demonstrating the characteristic lesions described above.

Epidemiologic Linkage Criteria for Diagnosis

An epidemiologic linkage can be established by close geographic and temporal proximity (in other words, part of the same mortality event) as one or more confirmed cases of SCTLD or at a site with a recent history of confirmed SCTLD with similar presentation as described in the “Case Definition Criteria” sections.